The present invention relates to an electrolysis cell for electrosynthesis, in an organic medium, of organic or organometallic compounds, containing two electrodes, one and only one of which is sacrificed during the electrosynthesis by the electrochemical reaction of which it forms the seat.
U.S. Pat. Nos. 3,573,178 and 3,141,841 describe the synthesis of tetraethyl lead in an electrolysis cell containing an anode which consists of lead balls and which is separated from the cylindrical cathode by means of an insulating porous side. Balls are added during the electrolysis to replace those which are consumed. However, the functioning of this device is unsatisfactory for strongly reducing metals such as magnesium, aluminum, zinc and titanium, which are covered with an insulating oxide coat which increases the contact resistance between grains significantly. Moreover, the granular form of these metals is sometimes expensive. Additionally, metallic dusts and slimes are often formed, which interferes with the operation.
South African Pat. No. 6,806,413 describes the synthesis of tetraethyl lead in an electrolysis cell containing a sacrificial anode which is in the form of a metal ribbon which runs between two cathodes in the form of discs. This system has a certain number of disadvantages. The thickness of the anode must especially be small so that the interelectrode distance remains constant; the rate of advance of the anode must therefore be rapid, and, in order to avoid the rupture of the ribbon, the device requires a relatively complicated mechanical system.
Moreover, several mechanical devices, often very complicated, which enable the distance between the electrodes to be adjusted so as to maintain it constant or which enable the worn-out anodes to be replaced, are known. For example, German Pat. No. 2,107,305 describes such a device.
Electrolysis cells containing a sacrificial anode have already been described for the electrosynthesis of oxalic acid from carbon dioxide, with aluminum on the one hand, in Chim. Ind. (Milan) 55. (1973) 156, and with zinc on the other, in J. Appl. Electrochem. 11 (1981) 743, for the electrocarboxylation of ethylene (Tetrahedron Lett. 1973, 3025) and for that of thioethers (German Democratic Republic Pat. No. 203,537).
These cells are without a diaphragm and generally have a coaxial cylindrical symmetry. In some cases, the central electrode functions as the sacrificial anode (for example, metal rod); in other cases, it functions as the cathode (for example, graphite). These laboratory cells do not easily lend themselves to an industrial use, especially in a continuous fashion, because they require frequent and not very convenient renewal of the anode on the one hand, and the distance between the 2 electrodes varies with time on the other.
The object of the present invention is to provide an electrolysis cell suitable for simple continuous industrial use, which has the advantages of the abovementioned industrial cells, viz. especially the maintenance of a constant gap between the electrodes, without having the disadvantages thereof.
The electrolysis cell according to the invention for the electrosynthesis, in an organic medium, of organic or organometallic compounds, containing two electrodes, only one of which is sacrificed during the electrosynthesis by the electrochemical reaction of which it forms the seat, is characterized in that:
the sacrificial electrode consists of at least one solid block of metal and is applied, under the influence of its own weight, against the other electrode from which it is separated by an electrical insulating material which allows the passage of the electrolytic solution and of which the shape and the dimensions enable the active surfaces of the two electrodes to remain parallel during the electro-synthesis,
the active surface of the unsacrificial electrode has, in all its points, a constant inclination relative to a direction D forming an angle less than 45 degrees with the vertical on the one hand, and an inclination less than 45 degrees relative to the vertical, on the other,
any straight line in the direction D passing through any point of the sacrificial electrode passes through the active surface of the unsacrificial electrode.
The inclination, at a point on the surface, relative to a direction D is normally considered to be the angle formed by the plane tangent to the surface at this point and by the straight line which has the direction D passing through this point.
A direction may be marked by an infinity of parallel straight lines.
Preferably, the direction D relative to which the surface of the unsacrificial electrode has a constant inclination is the vertical direction. In this preferred case, in which the D and the vertical directions are confounded, the angle formed by these two directions is zero.
Any point of the sacrificial electrode means a point situated on the surface of as well as within the solid metal blocks(s) forming this electrode.
The cell according to the invention has many advantages. First of all, it enables a constant and preferably small (less than 5 mm) gap to be maintained between the two electrodes throughout the period of electrolysis, which is very important in an organic medium which is not very conducting, in order to avoid an excessive electricity consumption and an excessive heating by the Joule effect.
As one of the two electrodes being gradually sacrificed during the electrochemical reaction, a means which makes it possible to maintain the distance between the two electrodes constant is necessarily required, and this is obtained within the scope of this invention, by virtue of the particular design and geometry of the cell. Additionally, it should be possible to replace the sacrificial electrode readily as soon as it is completely sacrificed, or preferably, for continuous processes, progressively as it is sacrificed, without stopping and interrupting the electrolysis.
The cell according to the invention enables the sacrificial electrode to be replaced very easily, without stopping the electrolysis by the superimposition of one (or more) other block on the solid metal block(s) which form(s) the sacrificial electrode, which is a considerable advantage when the processes are to be employed continuously. Furthermore, the entire electrode is sacrificed, without waste or loss. The cell according to the invention also makes it possible to use sacrificial electrodes which are solid, and therefore not very bulky for a given mass, and of different shapes. This is of great value from an economic point of view.
Another advantage is the fact that, taking into account the geometry of the cell and especially the inclination of the unsacrificial electrode, ground space requirement is much reduced, which results in a space saving, much appreciable from an economic point of view.
In many cases, the sacrificial electrode is the anode (anodic oxidation) as in the examples which will follow, but sometimes the sacrificial electrode is the cathode as in the case of the electrosynthesis of tetramethyl lead in an acetonitrile medium from methyl bromide using a lead cathode according to HE. Ulery JECS 116, 1201, 1969: EQU 4 CH.sub.3 Br+Pb (cathode)+4e.fwdarw.(CH.sub.3).sub.4 Pb+4 Br.
The sacrificial electrode consists of at least one solid metal block. The metal is preferably chosen from the group consisting of magnesium, aluminum, zinc and their alloys, viz. any alloy containing at least one of the three metals mentioned above. Many other metals, such as especially copper, nickel and lead, are also suitable. The choice of the metal depends, among other things, on the compound to be synthesized. In the case of the electrosynthesis of organometallic derivatives, the sacrificial electrode consists, for example, of the corresponding metal or an alloy based on this metal.
In the case of the electrosynthesis of carboxylic acids by the reduction of organic halides in the presence of CO.sub.2, magnesium will be preferred. For the electrosynthesis of alcohols by the electrochemical reduction of organic halides in the presence of carboxylated derivatives as well as for the electrosynthesis of ketones and aldehydes by the electrochemical reduction of organic halides in the presence of organic acid anhydrides, a metal chosen from the group consisting of magnesium, zinc, aluminum and their alloys will be preferred.
The solid metal blocks may be, for example, casting ingots of which the cross-section is square, or rectangular, or trapezoidal, or circular, or of any other shape. They may, if required, be machined before use so that their geometry is adapted to that of the unsacrificed electrode. Preferably, but without being of an imperative nature, such a machining is carried out in order to facilitate the start of the electrolysis.
According to a preferred variation, the sacrificial electrode consists of stacked solid metal blocks, each layer of the stacking containing only a single block. According to another variation, at least one layer of the stacking contains several blocks arranged side by side.
The sacrificial electrode is applied under the influence of its own weight, by gravity, against the other, unsacrificial electrode. According to a preferred variation, the sacrificial electrode is applied against the other electrode under the sole influence of its own weight. According to another variation, the sacrificial electrode is applied against the other electrode under the influence, in addition to that of its own weight, of that of an inert load resting on the sacrificial electrode. Preferably, the inert load is an electrical conductor and also serves for ensuring the electricity supply to the sacrificial electrode.
According to another variation, the sacrificial electrode is applied against the other electrode under the influence, in addition to that of its own weight, of the force produced by a spring which is compressed between the upper part of the sacrificial electrode and one side of the cell.
According to another preferred variation, the geometry of the unsacrificial electrode is such that it alone ensures the retention of the sacrificial electrode, i.e. no other side of the cell is used for this purpose. This is the case, for example, when the active surface of the unsacrificial electrode is conical or dihedral. These two preferred variations are described later (FIGS. 1 to 4).
According to another variation, the retention of the sacrificial electrode may also be ensured by the unsacrificial electrode and by an inert side of the cell at the same time. This is the case, for example, when the active surface of the unsacrificial electrode is in the form of a plane surface forming a dihedron with an inert side of the cell. This variation is also described later (FIG. 5).
The unsacrificial electrode is made of a conducting material. Metals such as iron, aluminum and nickel, alloys such as stainless steel, metal oxides such as Pbo.sub.2 and NiO.sub.2, and graphite may be mentioned in a non-limiting way. Preferably it is made of a metal chosen from the group consisting of nickel and stainless steel.
Preferably, the distance between the active surfaces of the two electrodes is less than 5 mm. This distance is typically measured along a common perpendicular, between the two parallel surfaces.
The two electrodes are separated by an electrical insulating material which allows the passage of the electrolytic solution and of which the shape and the dimensions enable the active surfaces of the 2 electrodes to remain parallel during the electrosynthesis. This electrical insulating material must, of course, have a mechanical strength adequate to support the sacrificial electrode which rests on this material.
Preferably, the electrical insulating material is a plastic material in the form of a grid, the thickness of which is less than 5 mm and the meshwork of which consists of two parallel wire networks, these two networks being superimposed, crossed, joined to each other at the points of contact between the wires, the thickness of the wires of each network being the same. In general, the two networks are joined to each other by soldering and the wires of the two networks have the same thickness.
By way of indication, the distance between the wires of each network is between a few millimetres and a few centimetres.
The wires of each network need not be parallel; their thickness does not need to be constant provided that, after assembling the networks, the grid has a constant maximum thickness at several points, less than approximately 5 mm.
The cross-section of the wires can be of any shape, for example square, rectangular, circular, elliptical or trapezoidal.
The plastic material may be made of, for example, polypropylene, polyethylene or polytetrafluoroethylene.
Such plastic grids have a high frequency of gaps, which allows a ready circulation of the electrolytic solution between the two electrodes on the one hand, and a relatively small area of contact with the electrodes, which avoids an excessive drop in their active surface, on the other.
As other materials separating the two electrodes, a cloth, a linen or a porous material of constant thickness such as, for example, a ceramic piece of a felt, may be used, within the scope of the present invention.
The renewal of the electrolytic solution between the electrodes may be, for example, ensured by a mechnical stirrer or by forced circulation, for example by means of a pump.
During electrolysis, the active surface of the sacrificial electrode facing the active surface of the other electrode is dissolved. Therefore, the sacrificial electrode lowers gradually, by gravity, under the simple influence of its own weight. Furthermore, as the dissolution is more intense at the points closest to the unsacrificial electrode, the sacrificial electrode has a tendency to adapt itself closely to the shape of the unsacrificial electrode, which reduces the risks of irregular dissolution.